Gain medium with rod or slab geometries are commonly used for high power solid-state laser. In general, slab geometry has advantages over rod geometry. For example, material for the slab is selected to have a relatively high index of refraction. The slab is cooled with a cooling medium having a relatively low index of refraction. This change in the index of refraction at the slab coolant interface results in incident light beams directed to one end face of the slab being totally internally reflected through the slab in a zigzag manner. The zigzag optical path averages the primary thermal gradient in the thickness direction, and reduces thermal lensing effect. In addition, the rectilinear cross section of the slab makes stress-induced birefringence much lower. Examples of solid-state lasers utilizing such zigzag amplifiers are disclosed in U.S. Pat. Nos. 4,730,324, 4,852,109, 5,305,345, 6,094,297 and 6,134,258.
In order to excite the laser slab to a relatively high-energy meta-stable state, various pumping methods have been developed for slab lasers, such as side pumping, end pumping, and edge pumping. For side-pumping geometry, the pumping sources are configured such that the light from the pumping source is directed along a lateral face of the slab in a direction generally perpendicular to the longitudinal axis of the slab, to obtain uniform pump power distribution in the slab. Examples of optical amplifiers with such configuration are disclosed in U.S. Pat. Nos. 4,127,827, 4,852,109, 5,271,031, 5,305,345, 5,646,773 and 5,651,021. Unfortunately, such configurations limit the absorption length of the pumping light to just a few millimeters. When such side pump configurations are used with lasers or optical amplifiers which use a solid-state laser material with relatively low absorption coefficient, such as Yb doped materials, it causes low absorption efficiency, and thus the overall efficiency are reduced. As disclosed in U.S. Pat. No. 6,094,297, a novel end-pumped zigzag slab laser which has relatively long absorption length is invented, but the disadvantage of this invention is that, with such configuration, power scaling is difficult because one can not couple more pump power into the slab while maintaining the slab thickness not too large for the reason of effective cooling. As disclosed in U.S. Pat. No. 6,134,258, a transverse-pumped configuration is invented in order to increase absorption length by using width of the slab instead of thickness to absorb pump light. Unfortunately, for high power laser using quasi-three-level lasing material, such as Yb doped materials, the doping concentration must be low in order to decrease laser threshold, therefore, absorption is not sufficient while utilizing such transverse-pumped configuration due to relatively low absorption coefficient.